1. Field of the Invention
The present invention relates to a fixing device for an electrophotographic image forming apparatus such as an electrophotographic copier or printer.
2. Prior Art
In a conventional electrophotographic image forming apparatus such as an electrophotographic copier or printer, a latent image of an original image is formed by uniformly charging a photosensitive member and exposing the original image on the photosensitive member. The formed latent image is developed with a toner to form a toner image of the original image, which is transferred onto a recording medium or transferred onto an intermediate transfer member and then further transferred onto the recording medium. The transferred toner image is subjected to a heat fixing process performed by a fixing device, whereby image formation is accomplished.
Examples of the fixing device include a fixing device of fixing-roller type and a fixing device of fixing-belt type. In the fixing device of fixing-roller type, a heating roller internally provided with a heat source and a pressing roller are disposed in opposing and contact relation. A fixing process is performed by causing a recording medium having a toner image transferred thereon to pass through a nip portion between the heating roller and the pressing roller and thereby fixing a toner image.
The aforementioned conventional fixing device of fixing-roller type has the disadvantage of a long waiting time from the time of power-on until a temperature at which a fixing process can be performed is reached in addition to the problems of large energy consumption for constantly holding the heating roller at a high temperature and temperature elevation within the fixing device due to heat dissipation from the fixing device into an image forming apparatus even during standby.
In the fixing device of fixing-belt type, on the other hand, the fixing belt is entrained in spanning relation between a heating roller internally provided with a heating source and a winding roller such that the heat of the heating roller is transmitted to the fixing belt. A fixing process is performed by causing a recording medium having a toner image transferred thereon to pass through a nip portion between a pressing roller disposed in opposing relation to the winding roller and the fixing belt and thereby heat pressing the toner image. Accordingly, it is no more necessary to provide a heat source at the nip portion, specifically within the fixing roller, which has been provided in the conventional fixing device and to effect heat conduction from inside the fixing roller, which has been effected conventionally. This allows a low-hardness elastic layer with a low heat conductivity to be provided at the nip portion and the provision of the low-hardness elastic layer ensures the provision of the nip portion with a large width.
An example of such a belt-type fixing device is disclosed in Japanese Patent Application Laid-open No. 137306/1996. The technology disclosed therein disposes a fixing belt in entrained and spanning relation between two rotating rollers, opposing an electromagnetic induction coil to the belt entrained in spanning relation between the rollers, and directly heats the belt with the electromagnetic induction coil. Since the rollers revolve the belt on receiving the tension of the belt, they should have a relatively large strength, which increases the heat capacity of the rollers. The heat given to the belt partly flows to the roller. Because of the large heat capacity, the heat of the belt is partly taken by the rollers so that the reduction of a warm-up time is approaching a limit even if efficient heating is performed by electromagnetic induction.
As disclosed in Japanese Patent Application Laid-open No. 107961/1993, on the other hand, an approach to reducing the warm-up time by bringing a heater into sliding contact with an outer surface of a heating roller and thereby performing heating has been made. However, the approach has the problem of low durability since the surface in sliding contact with the heater is a fixing surface and therefore is prone to flaws.
Another approach to direct heating performed by disposing a non-rotating heater at the nip portion has also been made. However, since a heater is required to have a pressing function and a heating function, the nip portion cannot have a sufficiently large width. In addition, the problem of high cost is also encountered since highly accurate dimensions and assembly are required of a heater holding member and the heater.
There has also been proposed a fixing device of fixing-belt type having a structure in which a non-rotating semi-cylindrical, i.e., trough-like heating plate is used in place of the heating roller and a fixing belt is entrained in spanning relation between the semi-cylindrical heating plate and a winding roller (see Japanese Patent Application Laid-open No. 343849/2001). Since the diameter of the non-rotating semi-cylindrical heating plate used in place of the heating roller corresponds to substantially half the diameter of the heating roller, the structure offers the advantages of a reduced lateral dimension of the fixing device and easy scaling down of the fixing device. Since a sheet-like heat generator as a heat source can be affixed directly to the inner surface of the trough-like heating plate, the structure also offers the advantages of high heat transmission efficiency, a reduced standby time, and the like.
In the foregoing fixing device of fixing-belt type using the semi-cylindrical heating plate, however, the inner surface of the semi-cylindrical heating plate is open so that heat is radiated uselessly. The radiated heat not only reduces heat efficiency but also renders the fixing device unsatisfactory in terms of safety. In inspecting the fixing device, the inspector may suffer a burn by mistake.
The present invention has been achieved to solve the foregoing problems and it is therefore an object of the present invention to provide a compact and low-cost fixing device of fixing-belt type which uses a heating plate in place of a heating roller to reduce a waiting time from the time of power-on until a temperature at which a fixing process can be performed is reached, ensure the supply of heat from the heating plate to the fixing belt, and give a proper tension to the fixing belt during a fixing operation.